Optical glass for precision press molding, preform for precision press molding, and process for the production thereof

ABSTRACT

A high-refractivity high-dispersion optical glass for producing an optical element, which requires no machining, such as polishing or lapping, of an optical-function surface after precision press molding, containing B 2 O 3 , SiO 2 , La 2 O 3 , Gd 2 O 3 , ZnO, Li 2 O, ZrO 2  and Ta 2 O 5  as essential components, containing 0 to 1 mol % of Sb 2 O 3  as an optional component, substantially containing none of PbO and Lu 2 O 3 , having a glass transition temperature of 630° C. or lower, and (1) having a refractive index nd and an Abbe&#39;s number νd which satisfy all of the following relational expressions, 1.80&lt;nd≦1.90, 35&lt;νd≦50, and nd≧2.025−(0.005×νd) or (2) having an nd of greater than 1.85 and a νd of greater than 35.

TECHNICAL BACKGROUND

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical glass for precisionpress molding, a preform for precision press molding, an opticalelement, and processes for the production of the preform and the opticalelement. More specifically, the present invention relates to ahigh-refractivity low-dispersion optical glass which does not requiremachining of an optical-function surface such as polishing or lappingafter precision press molding thereof and which is used for producing anoptical element such as an ultra-precision aspherical lens, a precisionpress molding preform made of the optical glass, an optical element madeof the same, and processes for the production of the above preform andoptical element.

[0003] 2. Prior Art

[0004] In recent years, digital cameras have appeared, and as theintegration and function of machines and devices using an optical systemare rapidly enhanced, it is increasingly demanded to enhance theprecision of the optical system and to decrease the optical system inweight and size. For materializing the above demands, optical designingusing an aspherical lens is coming to be a mainstream. For stablysupplying a large volume of aspherical lenses made of a high-functionalglass at a low cost, therefore, attention is actively paid to a moldshaping technique of directly forming an optical surface by pressmolding without polishing and lapping, and demands for an optical glasshaving high functionality (e.g., high refractivity andlow-dispersion/high refractivity and high-dispersion) and being suitablefor mold-shaping are increasing year after year.

[0005] Precision press molding of glass is a technique of shaping aglass preform under pressure at a high temperature into a glass shapedarticle having a form and a surface accuracy of an end article or a formand a surface accuracy very close to those of an end article. The aboveprecision press molding enables the highly productive production ofshaped articles (molded articles) having a desired form. At present,therefore, the precision press molding is employed to produce opticalparts such as spherical lenses, aspherical lenses and diffractiongratings, and the like. For producing an optical glass part by precisionpress-molding, naturally, it is required to shape a glass preform underpressure at a high temperature as described above, so that a mold usedfor the pressing is exposed to a high temperature and that a highpressure is applied thereto. It is therefore suppressing the damage thatis may be caused on the mold itself and a release film provided on aninner surface of the mold by the high-temperature environment of thepress molding, it is desired to decrease the glass transitiontemperature Tg and sag temperature Ts of a gob preform for glass moldingsuch that they are as low as possible.

[0006] As an optical glass having high-refractivity low-dispersion(refractive index nd>1.8 and Abbe's number νd>35) optical constants,various glasses containing B₂O₃ and La₂O₃ are conventionally known. Forexample, such glasses are disclosed in JP-A-8-217484, JP-A-54-90218 andJP-A-62-100449.

[0007] However, the above optical glasses aim at an improvement indevitrification resistance, and there is therefore involved a problemthat expensive components such as Lu₂O₃, etc., are essential, or that alarge amount of Sb₂O₃ that is a harmful component is essentiallyincorporated, for improving such optical glasses in stability. Further,of glass compositions disclosed in the above Publications, compositionsthat can attain a refractive index nd>1.8 and an Abbe's number νd>35very useful for optical designing contain almost no ZnO or Li₂O that issaid to be effective for decreasing the glass transition temperature, sothat they have poor suitability to press molding.

[0008] As described above, there has not been proposed any optical glasswhich attains a refractive index nd>1.8 and an Abbe's number νd>35(provided that a range surrounded by three points (nd, νd)=(1.85, 35),(1.8, 45) and (1.8, 35) is excluded), or particularly, there has not yetbeen proposed any optical glass for precision press molding which hasoptical constants, a refractive index nd>1.85 and an Abbe's numberνd>35.

[0009] The reason therefor is presumably as follows. Generally, a glasshaving such optical constants has a large content of rare earth metaloxide component and has a low degree of stabilization againstdevitrification, so that it has been difficult to develop a compositionthat makes it possible to decrease the glass transition temperature to aregion in which the glass can be press molded economically.

SUMMARY OF THE INVENTION

[0010] Under the circumstances, it is an object of the present inventionto provide a high-refractivity low-dispersion optical glass which doesnot require machining of an optical-function surface, such as polishingor lapping, after precision press molding thereof and which is used forproducing an optical element, a precision press molding preform made ofthe above optical glass, an optical element made of the above glass, andprocesses for the production of the above preform and the above opticalelement.

[0011] For achieving the above object, the present inventors have madediligent studies and as a result, it has been found that the aboveobject can be achieved by an optical glass containing specificcomponents as essential components and having a glass transitiontemperature of a specific value or smaller and specific opticalconstants. On the basis of the finding, the present invention has beencompleted.

[0012] That is, the present invention provides

[0013] (1) an optical glass for precision press molding (to be referredto as “optical glass I” hereinafter) comprising B₂O₃, SiO₂, La₂O₃,Gd₂O₃, ZnO, Li₂O, ZrO₂ and Ta₂O₅ as essential components, containing 0to 1 mol % of Sb₂O₃ as an optional component, substantially containingnone of PbO and Lu₂O₃, having a glass transition temperature of 630° C.or lower, and having a refractive index nd and an Abbe's number νd whichsatisfy all of the following relational expressions,

1.80<nd≦1.90

35<νd≦50,

and

nd≧2.025−(0.005×νd),

[0014] (2) an optical glass for precision press molding (to be referredto as “optical glass II” hereinafter) comprising B₂O₃, SiO₂, La₂O₃,Gd₂O₃, ZnO, Li₂O, ZrO₂ and Ta₂O₅ as essential components, containing 0to 1 mol % of Sb₂O₃ as an optional component, substantially containingnone of PbO and Lu₂O₃, having a glass transition temperature of 630° C.or lower, and having a refractive index nd of greater than 1.85 and anAbbe's number νd of greater than 35,

[0015] (3) an optical glass (to be referred to as “optical glass III”hereinafter) comprising, as essential components and by mol %, 15 to 40%of B₂O₃, 3 to 25% of SiO₂, 5 to 20% of La₂O₃, 5 to 20% of Gd₂O₃, 2 to35% of ZnO, 0.5 to 15% of Li₂O, 0.5 to 15% of ZrO₂ and 0.2 to 10% ofTa₂O₅, containing 0 to 15% of WO₃, 0 to 8% of Y₂O₃, 0 to 8% of Yb₂O₃ and0 to 1% of Sb₂O₃ as optional components, and further containing Nb₂O₅,BaO and GeO₂ as optional components, the total content of the abovecomponents being at least 95%, the optical glass substantiallycontaining none of PbO and Lu₂O₃, having a glass transition temperatureof 630° C. or lower, and having a refractive index nd and an Abbe'snumber νd which satisfy all of the following relational expressions,

1.80<nd≦1.90

35<νd≦50,

and

nd≧2.025−(0.005×νd),

[0016] (4) an optical glass as recited in the above (1), (2) or (3),which contains La₂O₃, Gd₂O₃, Yb₂O₃, Y₂O₃ and Sc₂O₃, the total content ofLa₂O₃, Gd₂O₃, Yb₂O₃, Y₂O₃ and Sc₂O₃ being 12 to 32 mol %, the molarratio of the content of La₂O₃ to said total content being 0.35 to 0.66,

[0017] (5) a preform for precision press molding which is made of theoptical glass recited in any one of the above (1) to (4),

[0018] (6) an optical element which is made of the optical glass recitedin any one of the above (1) to (4),

[0019] (7) a process for the production of a preform for precision pressmolding, which comprises flowing a molten glass made of the opticalglass recited in any one of the above (1) to (4) from a flow pipe,isolating molten glass having a predetermined weight, and shaping theisolated molten glass having the predetermined weight while the isolatedmolten glass is in a softened state, and

[0020] (8) a process for the production of an optical element whichcomprises heating a preform made of an optical glass to soften thepreform and producing the optical element from the softened preform byprecision press molding, said preform being the preform recited in theabove (5) or the preform produced by the method recited in the above(7).

BRIEF DESCRIPTION OF DRAWINGS

[0021]FIG. 1 is a graph showing a range of the refractive index nd andAbbe's number νd that one embodiment of the optical glass of the presentinvention has.

[0022]FIG. 2 is a schematic cross-sectional view of one example of aprecision press molding apparatus used in Examples.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The “press molding” in the present invention refers to a pressmolding method in which a glass material is heated to bring it into apress-moldable state, and press-shaping the glass material into aproduct by means of a press mold thereby to precisely transfer a moldingsurface of the press mold to the glass material that is in the abovestate, whereby the product (end article) can be produced withoutapplying machining such as polishing and lapping, etc., to the moldedproduct after the press molding. The press molding is generally appliedto the formation of optical elements (e.g., a lens, prism, and thelike). In the precision press molding of an optical element, forexample, the molding surface of a press mold is precisely transferredthereby to form an optical-function surface (a surface that performs anoptical function like a surface which transmits or reflects light (beam)to be controlled in an optical element), so that the thus-formedoptical-function surface can be allowed to exhibit performances as anoptical-function surface without machining the optical-function surfaceafter the press molding. The method of press-molding an optical elementby the above method is generally called “mold optics shaping”, and themethod of precision press molding of an aspherical lens is particularlyan excellently productive method since it is not required to polish orlap an optical-function surface into an aspherical surface.

[0024] The precision press molding is a method in which an articlerequired to have a high surface accuracy and internal quality such as anoptical element can be mass-produced highly productively. However, theglass to which the above method can be applied is limited to a glassthat can undergo plastic deformation at a relatively low temperature.When a glass having a high glass transition temperature is used, themolding surface of a press-shaping mold is exposed to a high temperatureduring the precision press molding, so that the above molding surface isintensely worn or broken. In the precision press molding, even a fineflaw that occurs on the molding surface of a press-shaping mold istransferred to the optical-function surface of an optical element thatis an end article, which means that the function of the optical elementis impaired. The glass transition temperature of the glass that isusable is therefore limited to 630° C. or lower.

[0025] The optical glass of the present invention includes threeembodiments, the optical glass I, the optical glass II and the opticalglass III. The optical glass I will be explained first.

[0026] The optical glass I of the present invention is an optical glasscontaining B₂O₃, SiO₂, La₂O₃, Gd₂O₃, ZnO, Li₂O, ZrO₂ and Ta₂O₅ asessential components, containing 0 to 1 mol % of Sb₂O₃ as an optionalcomponent, substantially containing none of PbO and Lu₂O₃, having aglass transition temperature of 630° C. or lower, and having arefractive index nd and an Abbe's number νd which satisfy all of thefollowing relational expressions,

1.80<nd≦1.90

35<νd≦50,

and

nd≧2.025−(0.005×νd),

[0027] The reason why the glass transition temperature of the opticalglass I is limited to 630° C. or lower is as already explained. Further,the optical glass I is required to be a high-refractivity low-dispersionoptical glass having a refractive index nd and an Abbe's number νd whichsatisfy all of the above three relational expressions. When the aboverequirement is illustrated in FIG. 1, nd and νd exist in a region thatis indicated by slanting lines but does not include nd 1.80 and νd 35.In FIG. 1, the axis of abscissas shows Abbe's number νd, and the axis ofordinates shows a refractive index nd.

[0028] The contents of the essential components in the above opticalglass I are not specially limited so long as there can be obtained anoptical glass having a glass transition temperature of 630° C. or lowerand having an refractive index nd and an Abbe's number νd which satisfyall of the above three relational expressions. However, the contents ofthe essential components are preferably the same as those in the opticalglass III to be described later. The function of each essentialcomponent will be explained later with regard to the optical glass III.

[0029] In the optical glass I, Sb₂O₃ as an optional component is used asan refining agent, and when it is used in an amount of 1 mol % or less,a sufficient effect can be obtained. Further, when the content of Sb₂O₃is large, the molding surface of the press-shaping mold may be damagedduring precision press molding. The content of Sb₂O₃ is thereforelimited to 1 mol % or less.

[0030] Further, the optical glass I substantially contains none of PbOand Lu₂O₃. The above “substantially containing none of PbO and Lu₂O₃”means that the optical glass I contains none of these substances thatare intentionally incorporated other than those included as impurities.Generally, the precision press molding is carried out in a non-oxidizingatmosphere such as a nitrogen atmosphere for protecting the moldingsurface of a press-shaping mold. PbO is a component that can be easilyreduced, so that the surface of a molded product comes to be cloudy dueto a deposit formed by reduction during the precision press. Further,since PbO is environmentally detrimental, PbO is excluded. Lu₂O₃ is notgenerally frequently used as a component for a glass as compared withother components. Further, Lu₂O₃ is a substance having a high scarcityvalue and is expensive for a raw material for an optical glass, so thatit is not any component whose use is desirable. The optical glass I ofthe present invention has stability as a glass having the aboveproperties, so that an unnecessary Lu₂O₃ is excluded.

[0031] The optical glass II of the present invention is a glasscontaining B₂O₃, SiO₂, La₂O₃, Gd₂O₃, ZnO, Li₂O, ZrO₂ and Ta₂O₅ asessential components, containing 0 to 1 mol % of Sb₂O₃ as an optionalcomponent, substantially containing none of PbO and Lu₂O₃, having aglass transition temperature of 630° C. or lower, and having arefractive index nd of greater than 1.85 and an Abbe's number νd ofgreater than 35.

[0032] The reason why the glass transition temperature of the opticalglass II is limited to 630° C. or lower is as already explained.Further, the optical glass II is required to be a high-refractivitylow-dispersion glass having a refractive index nd of greater than 1.85and an Abbe's number νd of greater than 35.

[0033] The contents of the essential components in the above opticalglass II are not specially limited so long as there can be obtained anoptical glass having a glass transition temperature of 630° C. or lowerand having an refractive index nd of greater than 1.85 and an Abbe'snumber νd of greater than 35. However, the contents of the essentialcomponents are preferably the same as those in the optical glass III tobe described later. The function of each essential component will beexplained later with regard to the optical glass III.

[0034] Sb₂O₃ as an optional component in the optical glass II is asexplained with regard to the above optical glass I.

[0035] Further, the optical glass II substantially contains none of PbOand Lu₂O₃. These components are also as explained with regard to theabove optical glass I.

[0036] The optical glass III of the present invention is an opticalglass containing, as essential components and by mol %, 15 to 40% ofB₂O₃, 3 to 25% of SiO₂, 5 to 20% of La₂O₃, 5 to 20% of Gd₂O₃, 2 to 35%of ZnO, 0.5 to 15% of Li₂O, 0.5 to 15% of ZrO₂ and 0.2 to 10% of Ta₂O₅,containing 0 to 15% of WO₃, 0 to 8% of Y₂O₃, 0 to 8% of Yb₂O₃ and 0 to1% of Sb₂O₃ as optional components, further containing Nb₂O₅, BaO andGeO₂ as optional components, the total content of the above componentsbeing at least 95%, the optical glass substantially containing none ofPbO and Lu₂O₃, having a glass transition temperature of 630° C. orlower, and having a refractive index nd and an Abbe's number νd whichsatisfy all of the following relational expressions,

1.80<nd≦1.90

35<νd≦50,

and

nd≧2.025−(0.005×νd).

[0037] The reason why the glass transition temperature of the opticalglass III is limited to 630° C. or lower is as already explained.Further, the optical glass III is required to be a high-refractivitylow-dispersion glass having optical constants, refractive index nd andAbbe's number νd, which satisfy all of the above three relationalexpressions.

[0038] The above composition has been found on the basis of experimentalchemistry. The unit of the contents (%) to be discussed below is mol %.

[0039] B₂O₃ is a network-forming oxide and an essential component in theoptical glass (I, II and III) of the present invention. Particularlywhen a high-refractivity component such as La₂O₃ or Gd₂O₃ isincorporated in a large amount, it is required to use B₂O₃ as a mainnetwork-forming component for forming a glass. However, the content ofB₂O₃ exceeds 40%, the refractive index of the glass is decreased, andthe glass obtained is not suitable for obtaining a high-refractivityglass. When it is less than 15%, the glass has no sufficient stabilityagainst devitrification, and the meltability of the glass decreases, sothat the content of B₂O₃ is preferably 15 to 40%, more preferably 20 to37%.

[0040] SiO₂ is a component for forming a glass network like B₂O₃, andwhen a small amount of SiO₂ is incorporated into a glass containing alarge amount of La₂O₃ and Gd₂O₃ as a substitute for part of B₂O₃ that isa main component, SiO₂ causes the liquidus temperature of the glass todecrease, improves the glass in high-temperature viscosity and greatlyimproves the glass in stability. When the content of SiO₂ is less than3%, the above effects are hardly produced. When the content of SiO₂exceeds 25%, the refractive index of the glass decreases, and further,the glass transition temperature increases, so that precision pressmolding of the glass is difficult. The content of SiO₂ is thereforepreferably in the range of 3 to 25%, more preferably in the range of 5to 20%.

[0041] As described already, La₂O₃ is an essential component that causesthe refractive index to increase and improves the chemical durability ofthe glass without decreasing the stability of the glass againstdevitrification and without increasing the dispersion. However, when thecontent of La₂O₃ is less than 5%, no sufficient effect is produced, andwhen it exceeds 20%, the glass is greatly deteriorated in stabilityagainst devitrification. The content of La₂O₃ is therefore preferably inthe range of 5 to 20%, more preferably in the range of 7 to 18%.

[0042] Like La₂O₃, Gd₂O₃ works to improve the glass in refractivity andchemical durability without deteriorating the stability of the glassagainst devitrification and the low dispersion of the glass. However,when the content of Gd₂O₃ is less than 5%, no sufficient effect can beobtained. When it exceeds 20%, the stability of the glass againstdevitrification is deteriorated, and the glass transition temperatureincreases, so that the precision press molding of the glass isdifficult. The content of Gd₂O₃ is therefore preferably in the range of5 to 20%, more preferably 6 to 18%, still more preferably 7 to 18%.

[0043] In the glass of B₂O₃—SiO₂—La₂O₃—Gd₂O₃—ZnO—Li₂O—ZrO₂—Ta₂O₅,generally, the total content of La₂O₃+Gd₂O₃ is adjusted preferably to atleast 12%, more preferably to 12 to 32%, for maintaining highfunctionality of high-refractivity and low-dispersion (refractive indexnd>1.8 and Abbe's number νd>35).

[0044] The amount ratio of the content of La₂O₃ by mol % to the totalcontent of lanthanoid oxides Ln₂O₃ (Ln=La, Gd, Yb, Y, Sc) by mol % inthe glass, La₂O₃/ΣLn₂O₃, is preferably in the range of 0.35 to 0.66,more preferably 0.45 to 0.66. The reason therefor will be explainedbelow.

[0045] In the glass for precision press molding, it is required toincorporate Li₂O and the like, components that impart the glass withprecision press molding suitability, i.e., a low glass transitiontemperature but destabilize the glass. When the content of thelanthanoid oxides essential for high refractivity and low dispersion isincreased, molding of the glass comes to be impossible. Generally, theamount (ΣLn₂O₃) is therefore limited.

[0046] However, the present inventors have found the following.Lanthanoid oxides other than La₂O₃ are added such that the ratio of thecontent of La₂O₃ to the total content of lanthanoid oxides Ln₂O₃ is 0.35to 0.66, whereby a stable glass can be obtained while increasing theamount of the lanthanoid oxides, and the glass containing Li₂O, etc.,components that decrease the stability of the glass can be molded as aglass. Further, it has been found that maintaining the above ratioserves to decrease the liquidus temperature and to improve thehigh-temperature viscosity. That is, when the ratio of La₂O₃/ΣLn₂O₃ isin the range of 0.35 to 0.66, a remarkably stable glass can be obtainedas compared with a case where the total content ΣLn₂O₃ is constant andthe ratio of La₂O₃/ΣLn₂O₃ is larger. For the above reason, further, itis preferred to adjust the total content (ΣLn₂O₃) of La₂O₃, Gd₂O₃,Yb₂O₃, Y₂O₃ and Sc₂O₃ to 12 to 32%.

[0047] ZnO is an essential component that decreases the meltingtemperature, liquidus temperature and glass transition temperature ofthe glass and is indispensable for the adjustment of a refractive index.When the content of ZnO is less than 2%, the expected results abovecannot be obtained. When the content thereof exceeds 35%, the dispersionis large, the stability against devitrification deteriorates, and thechemical durability decreases. The content of ZnO is preferably in therange of 2 to 35%, more preferably in the range of 5 to 32%.

[0048] Li₂O is a component that decreases the glass transitiontemperature to a great extent without involving a great decrease inrefractive index or a decrease in chemical durability, as compared withany other alkali metal oxide component. Particularly, when Li₂O isincorporated in a small amount, the effect thereof is large for itsamount, and it is effective for adjusting thermal properties of theglass. When the content of Li₂O is less than 0.5%, it produces littleeffect. When it exceeds 15%, the stability of the glass againstdevitrification sharply decreases, and the liquidus temperature of theglass also increases. The content of Li₂O is therefore preferably in therange of 0.5 to 15%, more preferably 1 to 12%, still more preferably 2to 12%.

[0049] Both ZnO and Li₂O are components that decrease the glasstransition temperature, so that the total content of ZnO+Li₂O ispreferably adjusted to at least 10 mol %, more preferably to at least 15mol %.

[0050] ZrO₂ is incorporated as a component for attaininghigh-refractivity and low-dispersion properties. When a small amount ofZrO₂ is incorporated, it has the effect of improving thehigh-temperature viscosity and the stability against devitrification, sothat it is preferred to incorporate a small amount of ZrO₂. When thecontent of ZrO₂ is less than 0.5%, it produces little effect. When thecontent thereof exceeds 15%, the liquidus temperature sharply increases,and the stability against devitrification is deteriorated. Therefore,the content of ZrO₂ is preferably in the range of 0.5 to 15%, morepreferably 1 to 10%.

[0051] Ta₂O₅ is incorporated as a component for attaininghigh-refractivity and low-dispersion properties. When a small amount ofTa₂O₅ is incorporated, it has the effect of improving the glass inhigh-temperature viscosity and stability against devitrification, sothat it is preferred to incorporate a small amount of Ta₂O₅. When thecontent of Ta₂O₅ is less than 0.2%, it produces no effect. When itexceeds 10%, the liquidus temperature sharply increases, and thedispersion becomes large. Therefore, the content of Ta₂O₅ is preferablyin the range of 0.2 to 10%, more preferably 1 to 8%.

[0052] WO₃ is a component that is incorporated as required for improvingthe glass in stability and meltability and improving the glass inrefractivity. When the content of WO₃ exceeds 15%, the dispersionbecomes large, and the necessary low-dispersion property can be nolonger obtained. Therefore, the content of WO₃ is preferably 15% orless, more preferably 12% or less.

[0053] Y₂O₃, Yb₂O₃ and BaO are incorporated as component for attaininghigh-refractivity and low-dispersion properties. When it is incorporatedin a small amount, it improves the glass in stability and chemicaldurability. When the content of each of individual components exceeds8%, any one of these impairs the stability of the glass againstdevitrification to a great extent and increases the glass transitiontemperature and sag temperature. Each component is therefore preferablycontrolled such that the content thereof is 8% or less, more preferably,7% or less.

[0054] Nb₂O₅ is a component that is incorporated as required forimproving the glass in stability and refractivity. When the contentthereof exceeds 8%, the dispersion becomes large, and the necessarylow-dispersion property can be no longer obtained. Therefore, thecontent of Nb₂O₅ is preferably 8% or less, more preferably, 5% or less.

[0055] GeO₂ is a component that stabilizes the glass like SiO₂ andimparts the glass with a higher refractive index than SiO₂ does.However, GeO₂ is expensive and increases the dispersion, so that thecontent of GeO₂ is preferably 8% or less.

[0056] Further, Sb₂O₃, PbO and Lu₂O₃ are as explained with regard to theabove optical glass I.

[0057] When the optical glass has a composition containing the aboveoptional components in the above amounts as required, there can beobtained an optical glass having qualities and properties that areexplained to be preferred. Above all, the optical glass more preferablyhas a glass composition containing 20 to 37% of B₂O₃, 5 to 20% of SiO₂,7 to 18% of La₂O₃, 6 to 18% of Gd₂O₃, 5 to 32% of ZnO, 1 to 12% of Li₂O,1 to 10% of ZrO₂, 1 to 8% of Ta₂O₅, 0 to 12% of WO₃, 0 to 7% of Y₂O₃, 0to 7% of Yb₂O₃, 0 to 5% of Nb₂O₅, 0 to 7% of BaO, 0 to 8% of GeO₂ and 0to 1% of Sb₂O₃, the total content of La₂O₃ and Gd₂O₃ being 12 to 32%,La₂O₃/ΣLn₂O₃ being 0.45 to 0.66. the optical glass more preferably has aglass composition containing 20 to 37% of B₂O₃, 5 to 20% of SiO₂, 7 to18% of La₂O₃, 7 to 18% of Gd₂O₃, 5 to 32% of ZnO, 2 to 12% of Li₂O, 1 to10% of ZrO₂, 1 to 8% of Ta₂O₅, 0 to 12% of WO₃, 0 to 7% of Y₂O₃, 0 to 7%of Yb₂O₃, 0 to 5% of Nb₂O₅, 0 to 7% of BaO, 0 to 8% of GeO₂ and 0 to 1%of Sb₂O₃, the total content of La₂O₃ and Gd₂O₃ being 12 to 32%,La₂O₃/ΣLn₂O₃ being 0.45 to 0.66.

[0058] In the above composition, the above more preferred compositionand the above particularly preferred composition, it is important thatthe total content of the above components is at least 95% for obtainingthe desired optical properties and at the same time for maintaining thestability of the glass. The optical glass may contain other componentssuch as Na₂O, K₂O, CaO, SrO, TiO₂, Al₂O₃, Ga₂O₃, and the like in a totalcontent of 5% or less for adjusting properties of the glass.

[0059] In the above glass composition and the above more preferredcomposition, preferably, the total content of B₂O₃, SiO₂, ZnO, Li₂O,La₂O₃, Gd₂O₃, ZrO₂, Ta₂O₅, WO₃, Y₂O₃ and Yb₂O₃ is at least 95%, and morepreferably, the above total content is at least 99%. Still morepreferably, the above total content is 100%.

[0060] Desirably, the optical glass (any one of the optical glasses I,II and III) of the present invention does not contain anyenvironmentally detrimental elements such as cadmium, radioactiveelements such as thorium or toxic elements such as arsenic. Further,desirably, they do not contain any fluorine in view of volatilizationduring melting of the glass.

[0061] The optical glass (any one of the optical glasses I, II and III)of the present invention can be produced, for example, by formulatingmaterial compounds and melting, refining, stirring and homogenizing theformulated glass material according to a conventional method.

[0062] Further, a glass melt that gives any one of the optical glasses(I, II and III) of the present invention is allowed to flow into a40×70×15 mm mold made of carbon, allowed to gradually cool to a glasstransition temperature, then annealed at the glass transitiontemperature for 1 hour and allowed to cool to room temperature to obtaina glass. In this case, there is precipitated no crystal that isobservable through a microscope. As described above, the optical glass(any one of the optical glasses I, II and III) of the present inventionis excellent in stability.

[0063] The optical glass of the present invention is transparent in avisible light region and suitable for producing a lens, a prism andother optical elements.

[0064] A precision press molding preform made of the optical glass (anyone of the optical glasses I, II and III) of the present invention and amethod of preparing the same will be explained below.

[0065] The precision press molding preform refers to a pre-shaped glassmaterial to be precision press molded under heat. As is alreadyexplained, the precision press molding is a method in which anoptical-function surface is formed by press molding, whereby an opticalelement as an end article (final product) is produced without anypolishing or lapping. When no removing processing like polishing andlapping is not carried out in any other portion than theoptical-function surface of a precision press molded article, the weightof the preform is adjusted such that it is equivalent to the weight ofan end article (final product) . The weight of a precision press moldedarticle is also equivalent to this weight. When the weight of a preformis smaller than the weight of an end precision press molded article, theglass is not fully charged in the molding surface of a press-shapingmold during its precision press molding, and there is caused a problemthat no intended surface accuracy can be obtained or that the thicknessof a molded article is smaller than an intended thickness. Further, whenthe weight of a preform is larger than the weight of an end precisionpress molded article, there is caused a problem that excess glasspenetrate gaps of precision press mold members to form burrs, or that amolded article has a larger thickness than an intended thickness. It istherefore required to control the weight of a precision press moldingpreform more accurately than the weight of any general press moldingglass material that is finished by polishing or lapping after pressmolded. In the precision press molding preform, further, the surface ofthe preform is left on an end article as a press molded article surface,so that the surface of the preform is required to be free of a flaw andsoiling.

[0066] The method for producing the above precision press moldingpreform includes a method in which a molten glass is allowed to flow, amolten glass gob having a predetermined weight is separated, and themolten glass gob is shaped into a preform (to be referred to as “hotshaping method” hereinafter) and a method in which a molten glass iscast into a mold, a shaped glass is cooled, and the obtained glass gobis machined to a predetermined size (to be referred to as “cold shapingmethod” hereinafter).

[0067] In the hot shaping method, a molten glass which is prepared bymelting, clarification and homogenization and which has, for example, atemperature of approximately 1,000 to 1,400° C. and a viscosity ofapproximately 0.1 to 5 dPa·s is prepared, the temperature of the abovemolten glass is adjusted such that the molten glass has a viscosity ofapproximately 3 to 60 dPa·s, and the molten glass is allowed to flow outof a flow nozzle or a flow pipe, to shape it into a preform. The methodof adjusting the above temperature includes, for example, a method inwhich the temperature of the flow nozzle or the flow pipe is controlled.The flow nozzle or flow pipe is desirably made of platinum or a platinumalloy. The method of shaping the molten glass into a preformspecifically includes a method in which molten glass is dropped from theflow nozzle as a molten glass drop having a predetermined weight and themolten glass drop is received with a receiving member and shaped into apreform, a method in which the above molten glass drop having apredetermined weight is dropped from the above flow nozzle into liquidnitrogen and shaped into a preform, and a method in which a molten glassflow is allowed to flow down from the flow pipe made of platinum or aplatinum alloy, a forward end portion of the molten glass flow isreceived with a receiving member, a constricted portion is formed in amolten glass flow portion between the nozzle and the receiving member,molten glass flow is separated in the constricted portion, and a moltenglass gob having a predetermined weight is received with the receivingmember and shaped into a preform. When the molten glass is dropped, theglass preferably has a viscosity of 3 to 30 dPa·s. When the molten glassis flowed down as a molten glass flow, the glass preferably has aviscosity of 2 to 60 dPa·s.

[0068] The form of the preform can be determined by taking account ofthe form of precision press molded article. Examples of the form of thepreform preferably include a spherical form and an oval form. In the hotshaping method, a preform having a smooth surface can be easily obtainedsince the surface of the preform is formed when the glass has asoftening temperature or higher. Particularly, in a method in which amolten glass gob is shaped into a preform while floated above a shapingmold with air pressure, or a method in which a molten glass gob isplaced into, and shaped into a preform in, a medium prepared by coolinga substance that is a gas at an ordinary temperature under ordinarypressure into a liquid, there can be easily produced a preform having asmooth surface free of flaws, soling and surface alteration, forexample, a preform having a free surface.

[0069] In the cold shaping method, for example, the above molten glassprepared by melting, refining and homogenization is cast into a castingmold, shaped into the form of a glass block, the glass block isgradually cooled to decrease strain of the glass, then, the glass blockis machined or cut to prepare a glass gob having predetermineddimensions and a predetermined weight, and the glass gob issurface-smoothened to give a preform.

[0070] The method of precision press molding the above preform toproduce an optical element will be explained below.

[0071] The precision press molding uses a press-shaping mold having amolding surface that is accurately processed beforehand so as to have adesired form, and a release film may be formed on the molding surfacefor preventing the fusion of the glass during pressing. The precisionpress molding can be carried out by a known method including precisionpress molding in an atmosphere of a non-oxidizing gas such as nitrogengas for preventing damage of the molding surface, such as damage byoxidation.

[0072] In the above manner, various lenses such as a spherical lens, anaspherical lens, a micro lens, a lens array, a micro lens array, etc.,and optical elements such as a prism, a polygonal mirror, etc., can beproduced from the optical glass (any one of the optical glasses I, II,and III) of the present invention without machining theiroptical-function surfaces.

EXAMPLES

[0073] The present invention will be explained more specifically withreference to Examples hereinafter, while the present invention shall notbe limited to these Examples.

Examples 1-64

[0074] Oxides, carbonates, sulfates, nitrates, hydroxides, etc., such asSiO₂, H₃BO₃, La₂O₃, ZnO, ZnCO₃, ZrO₂, Li₂CO₃, etc., as raw materialswere provided, and 250 to 300 g of each of these components was weighedso as to form compositions shown in Tables 1 to 7. These raw materialsin each Example were fully mixed to prepare a formulated batch, theformulated batch was placed in a platinum crucible, and the formulatedbatch was melted in air in an electric furnace maintained at 1,200 to1,450° C., with stirring for 2 to 4 hours. After melted, the glass meltwas allowed to flow into a 40×70×15 mm mold made of carbon and allowedto cool to a glass transition temperature, and immediately thereafter,the glass was placed in an annealing furnace and annealed in a glasstransition temperature range for about 1 hour. Then, the glass in thefurnace was allowed to cool to room temperature, to give an opticalglass. In the thus-obtained optical glasses, there was precipitated nocrystal that was observable through a microscope.

[0075] Each optical glass was measured for properties according to thefollowing methods. Tables 1 to 7 shows the results.

[0076] (1) Refractive Index (nd) and Abbe's Number (νd)

[0077] An optical glass held at a temperature between Tg and Ts wastemperature-decreased at a temperature decrease rate of −30° C./hour,and the optical glass was measured for a refractive index (nd) and anAbbe's number (νd).

[0078] (2) Glass Transition Temperature (Tg) and Sag Temperature (Ts)

[0079] An optical glass was measured at a temperature elevation rate of4° C./minute with a thermomechanical analyzer supplied by Rigaku DenkiK.K. TABLE 1 Example 1 2 3 4 5 6 7 8 9 10 Glass composition (mol %) B₂O₃32.28 35.48 33.86 33.33 35.20 34.65 35.48 35.63 30.28 31.62 SiO₂ 9.459.68 9.45 9.52 9.60 9.45 9.68 9.72 9.56 9.49 La₂O₃ 9.06 9.27 7.87 9.139.20 9.06 8.06 8.10 10.36 9.49 Gd₂O₃ 9.06 9.27 7.87 9.13 9.20 9.06 8.068.10 10.36 9.49 ZnO 28.35 24.19 28.35 28.57 25.60 28.35 22.58 22.6722.31 23.72 Li₂O 3.94 4.03 3.94 3.97 4.00 2.36 5.65 5.67 5.58 5.53 ZrO₂4.72 4.84 4.72 3.17 4.80 4.72 6.45 5.67 3.98 3.95 Ta₂O₅ 3.15 3.23 3.153.17 2.40 2.36 4.03 3.64 2.79 1.98 WO₃ 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 4.78 4.74 Y₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 Yb₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Nb₂O₅ 0.000.00 0.79 0.00 0.00 0.00 0.00 0.81 0.00 0.00 BaO 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 (ΣLn₂O₃) (18.12) (18.54) (15.74) (18.26) (18.40) (18.12)(16.12) (16.20) (20.72) (18.98) (La₂O₃/ΣLn₂O₃) (0.50) (0.50) (0.50)(0.50) (0.50) (0.50) (0.50) (0.50) (0.50) (0.50) (Li₂O + ZnO) (32.28)(28.23) (32.28) (32.54) (29.60) (30.71) (28.23) (28.34) (27.89) (29.25)(La₂O₃ + Gd₂O₃) (18.12) (18.54) (15.74) (18.26) (18.40) (18.12) (16.12)(16.20) (20.72) (18.98) Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0100.0 100.0 100.0 Properties Tg(TMA)(° C.) 566 573 556 564 570 577 562561 565 556 Ts(° C.) 616 623 606 614 620 627 612 611 615 606 nd 1.817511.81014 1.81033 1.81032 1.81040 1.80680 1.80756 1.81001 1.84150 1.82528νd 43.95 44.51 43.26 44.40 44.50 44.97 43.77 43.20 40.85 41.79 Specificgravity 5.08 5.00 4.92 5.05 5.01 4.99 4.91 4.87 5.30 5.13

[0080] TABLE 2 Example 11 12 13 14 15 16 17 18 19 20 Glass composition(mol %) B₂O₃ 30.83 30.28 31.08 30.52 25.29 24.90 31.33 31.58 31.08 25.68SiO₂ 9.49 9.56 9.56 9.64 18.39 16.60 9.64 9.72 9.56 15.56 La₂O₃ 9.889.96 9.96 10.44 11.11 15.77 10.84 11.74 11.55 12.06 Gd₂O₃ 9.88 9.96 9.9610.44 11.11 9.54 10.84 11.74 11.55 12.06 ZnO 22.13 22.31 20.72 20.8819.92 8.30 20.88 17.81 20.72 20.23 Li₂O 5.53 5.58 5.58 5.62 2.30 9.964.02 4.05 2.39 2.33 ZrO₂ 4.74 3.98 5.58 4.02 3.83 3.32 4.02 4.05 3.983.89 Ta₂O₅ 1.98 3.59 2.79 3.61 3.45 4.15 3.61 2.83 2.79 3.50 WO₃ 5.534.78 4.78 4.82 4.60 5.81 4.82 6.48 6.37 4.67 Y₂O₃ 0.00 0.00 0.00 0.000.00 1.66 0.00 0.00 0.00 0.00 Yb₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 Nb₂O₅ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00BaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 (ΣLn₂O₃) (19.76) (19.92) (19.92)(20.88) (22.22) (26.97) (21.69) (23.48) (23.10) (24.12) (La₂O₃/ΣLn₂O₃)(0.50) (0.50) (0.50) (0.50) (0.50) (0.58) (0.50) (0.50) (0.50) (0.50)(Li₂O + ZnO) (27.67) (27.89) (26.29) (26.51) (22.22) (18.26) (24.90)(21.86) (23.11) (22.57) (La₂O₃ + Gd₂O₃) (19.76) (19.92) (19.92) (20.88)(22.22) (25.31) (21.68) (23.48) (23.10) (24.12)

100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 PropertiesTg(TMA)(° C.) 560 563 565 569 612 592 578 589 597 616 Ts(° C.) 610 613615 619 662 647 628 639 647 669 nd 1.83660 1.84383 1.84096 1.846851.84905 1.83226 1.85065 1.85595 1.85922 1.86308 νd 41.46 40.45 40.9240.52 40.72 39.97 40.53 40.15 40.02 40.50 Specific gravity 5.21 5.315.24 5.34 5.41 5.43 5.39 5.45 5.48 5.55

[0081] TABLE 3 Example 21 22 23 24 25 26 27 28 29 30 Glass composition(mol %) B₂O₃ 26.88 23.35 24.90 24.03 23.94 23.62 25.00 24.19 24.90 24.31SiO₂ 12.65 18.68 16.06 15.50 15.44 15.75 16.13 16.13 16.06 15.69 La₂O₃12.65 12.45 12.85 12.79 11.97 12.99 12.90 12.90 12.85 12.55 Gd₂O₃ 12.6512.45 12.85 12.79 11.97 12.99 12.90 12.90 12.85 12.55 ZnO 20.55 17.1214.46 20.16 20.08 17.32 12.90 12.90 14.46 14.12 Li₂O 2.37 3.89 6.43 2.332.32 3.94 6.45 7.26 6.43 6.27 ZrO₂ 3.95 3.98 4.02 3.88 5.41 4.72 4.844.84 4.02 3.92 Ta₂O₅ 3.56 3.50 3.61 3.88 4.25 3.94 4.03 4.03 2.81 1.18WO₃ 4.74 4.67 4.82 4.65 4.63 4.72 4.84 4.84 4.82 4.71 Y₂O₃ 0.00 0.000.00 0.00 0.00 1.66 0.00 0.00 0.00 0.00 Yb₂O₃ 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 Nb₂O₅ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 4.71 (ΣLn₂O₃) (25.30)(24.90) (25.70) (25.58) (23.94) (25.98) (25.80) (25.80) (25.70) (25.10)(La₂O₃/ΣLn₂O₃) (0.50) (0.50) (0.50) (0.50) (0.50) (0.50) (0.50) (0.50)(0.50) (0.50) (Li₂O + ZnO) (22.92) (21.01) (20.88) (22.48) (22.39)(21.26) (19.35) (20.16) (20.88) (20.39) (La₂O₃ + Gd₂O₃) (25.30) (24.90)(25.70) (25.58) (23.94) (25.98) (25.80) (25.80) (25.70) (25.10) Total100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 PropertiesTg(TMA)(° C.) 617 612 597 617 623 620 605 604 600 599 Ts(° C.) 665 662652 671 673 670 659 663 655 655 nd 1.87132 1.86218 1.86398 1.880851.87673 1.87726 1.86734 1.86865 1.86405 1.86841 νd 39.97 40.29 40.5139.82 39.19 39.64 40.04 39.92 40.10 38.82 Specific gravity 5.63 5.555.55 5.65 5.65 5.68 5.57 5.58 5.49 5.34

[0082] TABLE 4 Example 31 32 33 34 35 36 37 38 39 40 Glass composition(mol %) B₂O₃ 24.90 24.51 24.90 24.90 24.10 24.10 25.51 24.49 24.69 24.49SiO₂ 16.06 15.81 16.06 16.06 16.06 16.06 16.46 16.33 16.46 16.33 La₂O₃12.85 11.86 12.05 12.05 12.85 12.85 12.35 12.24 12.35 12.24 Gd₂O₃ 12.8511.86 12.05 12.05 12.85 12.85 12.35 12.24 12.35 12.24 ZnO 14.46 14.2314.46 14.46 14.46 14.46 9.88 11.43 9.88 9.80 Li₂O 6.43 6.32 6.43 6.436.43 6.43 9.05 8.98 9.05 8.98 ZrO₂ 4.02 3.95 4.02 4.02 4.02 4.02 4.124.08 4.12 4.08 Ta₂O₅ 3.61 3.56 3.61 3.61 3.61 3.61 3.70 3.67 4.53 3.67WO₃ 0.00 4.74 4.82 4.82 4.82 4.82 4.94 4.90 4.94 6.53 Y₂O₃ 0.00 0.001.61 0.00 0.80 0.00 1.65 1.63 1.65 1.63 Yb₂O₃ 0.00 0.00 0.00 1.61 0.000.80 0.00 0.00 0.00 0.00 Nb₂O₅ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 BaO 0.00 3.16 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂4.82 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 (ΣLn₂O₃) (25.70)(23.72) (25.71) (25.71) (26.50) (26.50) (26.35) (26.11) (26.35) (26.11)(La₂O₃/ΣLn₂O₃) (0.50) (0.50) (0.47) (0.47) (0.48) (0.48) (0.47) (0.47)(0.47) (0.47) (Li₂O + ZnO) (20.88) (20.55) (20.88) (20.88) (20.88)(20.88) (18.93) (20.41) (18.93) (18.78) (La₂O₃ + Gd₂O₃) (25.70) (23.72)(24.10) (24.10) (25.70) (25.70) (24.70) (24.48) (24.70) (24.48) Total100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 PropertiesTg(TMA)(° C.) 604 596 603 603 608 609 595 594 600 595 Ts(° C.) 662 653659 660 665 664 651 652 657 652 nd 1.86574 1.85564 1.86200 1.862581.86818 1.86809 1.85642 1.85949 1.86360 1.86420 νd 40.42 40.40 40.3240.25 40.27 40.19 40.82 40.60 40.01 39.68 Specific gravity 5.40 5.505.50 5.60 5.57 5.63 5.44 5.47 5.51 5.50

[0083] TABLE 5 Example 41 42 43 44 45 46 47 48 49 50 Glass composition(mol %) B₂O₃ 25.10 24.69 24.49 24.20 24.49 24.59 25.00 25.10 25.51 25.10SiO₂ 16.74 16.46 16.33 16.13 16.33 16.39 16.39 16.19 16.46 16.19 La₂O₃12.55 12.35 12.24 15.32 17.14 15.57 15.57 15.38 15.64 14.57 Gd₂O₃ 12.5512.35 12.24 8.87 7.35 9.43 9.02 8.91 9.05 11.34 ZnO 5.05 9.88 9.80 9.689.80 9.84 9.84 12.96 9.88 12.96 Li₂O 11.72 9.05 8.98 8.87 8.98 9.02 9.027.29 9.05 7.29 ZrO₂ 4.18 4.12 4.08 4.03 4.08 3.28 3.28 3.24 3.29 4.05Ta₂O₅ 3.77 4.53 3.67 3.63 3.67 3.69 3.69 3.64 3.70 3.64 WO₃ 6.69 4.946.53 6.45 6.53 6.56 6.56 5.67 5.76 4.86 Y₂O₃ 1.67 0.00 0.00 2.81 1.631.64 0.00 1.62 1.65 0.00 Yb₂O₃ 0.00 1.65 1.63 0.00 0.00 0.00 1.64 0.000.00 0.00 Nb₂O₅ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 BaO0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 .00 (ΣLn₂O₃) (26.79) (26.35) (26.11)(27.00) (26.12) (26.64) (26.23) (25.91) (26.34) (25.91) (La₂O₃/ΣLn₂O₃)(0.47) (0.47) (0.47) (0.57) (0.66) (0.58) (0.59) (0.59) (0.59) (0.56)(Li₂O + ZnO) (16.74) (18.93) (18.78) (18.55) (18.78) (18.85) (18.85)(20.24) (18.93) (20.24) (La₂O₃ + Gd₂O₃) (25.10) (24.70) (24.48) (24.19)(24.49) (25.00) (24.59) (24.29) (24.69) (25.91) Total 100.0 100.0 100.0100.0 100.0 100.0 100.0 100.0 100.0 100.0 Properties Tg(TMA)(° C.) 597599 597 598 591 594 590 594 589 597 Ts(° C.) 640 655 653 655 646 649.6647 651 645 654 nd 1.85901 1.86382 1.86456 1.86702 1.86545 1.864351.86180 1.86093 1.85775 1.86212 νd 39.92 40.10 39.60 39.68 39.57 39.8339.86 40.17 40.31 40.44 Specific gravity 5.45 5.60 5.60 5.43 5.39 5.445.41 5.42 5.38 5.48

[0084] TABLE 6 Example 51 52 53 54 55 56 57 58 Glass composition (mol %)B₂O₃ 25.10 25.10 25.10 24.70 24.80 24.90 28.24 26.29 SiO₂ 16.19 16.1916.19 15.94 16.00 16.06 12.55 15.94 La₂O₃ 12.96 14.57 15.38 14.34 14.4014.46 10.20 11.16 Gd₂O₃ 11.34 9.72 8.91 9.56 9.60 9.64 10.20 11.16 ZnO12.96 12.96 12.96 14.34 13.60 12.85 23.53 19.12 Li₂O 7.29 7.29 7.29 7.177.60 8.03 5.49 7.17 ZrO₂ 3.24 3.24 3.24 3.19 3.20 3.21 5.49 3.98 Ta₂O₅3.64 3.64 3.64 3.59 3.60 3.61 2.74 3.59 WO₃ 5.67 5.67 5.67 5.58 5.605.62 1.57 1.59 Y₂O₃ 0.81 0.81 0.81 0.80 0.80 0.80 0.00 0.00 Yb₂O₃ 0.810.81 0.81 0.80 0.80 0.80 0.00 0.00 Nb₂O₅ 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 (ΣLn₂O₃) (25.92) (25.91) (25.91) (25.50)(25.60) (25.70) (20.40) (22.31) (La₂O₃/ΣLn₂O₃) (0.50) (0.56) (0.59)(0.56) (0.56) (0.56) (0.50) (0.50) (Li₂O + ZnO) (20.24) (20.24) (20.24)(21.51) (21.20) (20.88) (29.02) (26.29) (La₂O₃ + Gd₂O₃) (24.30) (24.29)(24.29) (23.90) (24.00) (24.10) (20.40) (22.32) Total 100.0 100.0 100.0100.0 100.0 100.0 100.0 100.0 Properties Tg(TMA)(° C.) 598 594 595 592592 589 579 567 Ts(° C.) 656 650 650 648 648 644 635 621 nd 1.860831.86147 1.86156 1.86164 1.86078 1.85964 1.83553 1.83327 νd 40.19 40.2240.15 40.04 40.32 40.11 42.80 42.78 Specific gravity 5.52 5.49 5.47 5.495.48 5.47 5.29 5.25

[0085] TABLE 7 59 60 61 62 63 64 Glass composition (mol %) B₂O₃ 31.3330.28 30.16 29.92 29.92 30.95 SiO₂ 9.64 11.16 11.11 11.02 11.02 9.52 ZnO20.88 20.72 21.43 22.05 22.05 22.22 Li₂O 4.02 3.98 3.97 3.94 3.94 3.97La₂O₃ 13.25 13.94 13.49 12.99 12.99 13.10 Gd₂O₃ 8.43 7.57 7.14 6.69 6.696.75 ZrO₂ 4.02 3.98 3.97 3.94 4.72 4.76 Ta₂O₅ 3.61 3.59 3.97 3.94 3.943.97 Nb₂O₅ 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.000.00 WO₃ 4.82 4.78 4.76 5.51 4.72 4.76 Y₂O₃ 0.00 0.00 0.00 0.00 0.000.00 Yb₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.000.00 (ΣLn₂O₃) (21.69) (21.51) (20.63) (19.69) (19.69) (19.84)(La₂O₃/ΣLn₂O₃) (0.61) (0.65) (0.65) (0.66) (0.66) (0.66) (Li₂O + ZnO)(24.90) (24.70) (25.40) (25.98) (25.98) (26.19) Total 100.0 100.0 100.0100.0 100.0 100.0 Tg(TMA)(° C.) 586 586 583 581 580 579 Ts(° C.) 637 636634 631 632 630 nd 1.85171 1.84790 1.84788 1.84739 1.84838 1.85052 νd40.50 40.45 40.18 39.76 40.17 40.01 Density 5.34 5.29 5.29 5.27 5.265.28

[0086] As shown above, there can be obtained optical glasses forprecision press molding, comprising B₂O₃, SiO₂, La₂O₃, Gd₂O₃, ZnO, Li₂O,ZrO₂ and Ta₂O₅, having a glass transition temperature of 630° C. orlower and having a refractive index nd and an Abbe's number νd whichsatisfy all of the following relational expressions,

1.80<nd≦1.90

35<νd≦50,

and

nd≧2.025−(0.005×νd).

Example 65

[0087] Precision press molding preforms were produced from the opticalglasses obtained in Examples 1 to 64 as follows. First, melting,refining and homogenization were carried out to obtain a molten glassthat was to give one of the above optical glasses, the molten glass wasdripped from a flow nozzle made of a platinum alloy toward a receivingmember, and in a concave portion of the receiving member, the receivedmolten glass drop was floated and rolled while a gas was ejected upwardfrom a gas ejection port formed in a bottom of the concave portion, toshape the molten glass into a preform (Method 1). The thus-formedpreforms from the above optical glasses had a weight equivalent to theweight of an intended end product, underwent no devitrification and hada smooth surface free of defects such as a flaw, soiling and alteration.

[0088] Separately, the same molten glass drop as above was dropped fromthe flow nozzle in the same manner as above into liquid nitrogen andshaped into a preform (Method 2). Like the above preforms, each of thethus-obtained preforms underwent no devitrification, had high weightaccuracy and had a smooth surface free of defects.

[0089] Further, the same molten glass as above was allowed to flow downfrom a flow pipe to form a molten glass flow, a lower end portion wasreceived with a receiving mold member, a constricted portion was formedsomewhere in the molten glass flow, and when a glass lower than theconstricted portion came to have a predetermined weight value, the glasswas separated in the constricted portion. The separated molten glass gobhaving the above weight was shaped into a preform with a receiving moldmember (Method 3). Like the preforms obtained by the above Methods 1 and2, each of the thus-obtained preforms underwent no devitrification, hadhigh weight accuracy and had a smooth surface free of defects.

Example 66

[0090] Each of the spherical preforms made of the optical glasses ofExamples 1 to 64 obtained by Method 1 in Example 65 was heated andprecision press molded (aspherical-precision pressed) with an apparatusshown in FIG. 2, to give aspherical lenses.

[0091] Particulars of the precision press molding were as follows. Thepreform 4 was placed between a lower mold member 2 and an upper moldmember 1 which had an aspherical form and were made of SiC, then theatmosphere inside a quartz tube 11 was replaced with a nitrogenatmosphere inside, and an electric current was applied to heater 12 toheat the inside of the quartz tube 11. The temperature inside theshaping mold was set at a temperature between a sag temperature of theglass+20° C. and the sag temperature of the glass+80° C., and while thistemperature was maintained, a press rod 13 was moved downward to pressthe upper mold member 1 thereby to press-mold the preform (molding glassgob) in the shaping mold. The press-molding was carried out under amolding pressure of 8 MPa for a molding time period of 30 seconds. Afterthe press-molding, the molding pressure was decreased, and while thepress-molded glass molded product was in contact with the lower moldmember 2 and the upper mold member 1, the molded product was graduallycooled to a temperature that was the glass transition temperature−30° C.Then, the molded product was rapidly cooled to room temperature. Then,the glass molded as an aspherical lens was taken out of the shapingmold, and subjected to measurement of a form and inspection of anappearance. The thus-obtained aspherical lenses from the optical glassesin Examples 1 to 64 had remarkably high accuracy.

[0092] In FIG. 2, numeral 3 indicates a sleeve, numeral 9 indicates asupport rod, numeral 10 indicates a support bed, and numeral 14indicates a thermocouple.

[0093] Aspherical lenses made of the optical glasses in Examples 1 to 64were obtained from the preforms prepared by Methods 2 and 3 in the samemanner as above. Like the above spherical lenses, the thus-obtainedspherical lenses had remarkably high accuracy. While the preforms usedin this Example had the form of a sphere and had a diameter of 2 to 30mm, the form and dimensions of the preform can be determined as requireddepending upon the form, etc., of a precision press molded product(article).

[0094] A press-shaping mold having a form suitable for producing an endproduct can give other lens or an optical element such as a prism or apolygonal mirror.

[0095] An anti-reflection film or an optical multi-layered film such asa high reflection film can be formed on the optical-function surface ofthe thus-obtained optical element as required.

EFFECT OF THE INVENTION

[0096] According to the present invention, there can be provided aprecision press molding optical glass that has high-refractivity andlow-dispersion properties and which is usable for the production of anoptical element such as an ultra-precision aspherical lens without anymachining, polishing or lapping, of the optical-function surface thereofafter precision press molding.

[0097] According to the present invention, further, there can be alsoprovided a precision press molding preform made of the above opticalglass and a process for the production thereof, and there can be furtherprovided an optical element made of the above optical glass and aprocess for the production of an optical element such as an asphericallens, or the like, highly productively from the above preform byprecision press molding.

What is claimed is:
 1. An optical glass for precision press molding,comprising B₂O₃, SiO₂, La₂O₃, Gd₂O₃, ZnO, Li₂O, ZrO₂ and Ta₂O₅ asessential components, containing 0 to 1 mol % of Sb₂O₃ as an optionalcomponent, substantially containing none of PbO and Lu₂O₃, having aglass transition temperature of 630° C. or lower, and having arefractive index nd and an Abbe's number νd which satisfy all of thefollowing relational expressions, 1.80<nd≦1.90 35<νd≦50, andnd≧2.025−(0.005×νd).
 2. An optical glass for precision press molding,comprising B₂O₃, SiO₂, La₂O₃, Gd₂O₃, ZnO, Li₂O, ZrO₂ and Ta₂O₅ asessential components, containing 0 to 1 mol % of Sb₂O₃ as an optionalcomponent, substantially containing none of PbO and Lu₂O₃, having aglass transition temperature of 630° C. or lower, and having arefractive index nd of greater than 1.85 and an Abbe's number νd ofgreater than
 35. 3. An optical glass comprising, as essential componentsand by mol %, 15 to 40% of B₂O₃, 3 to 25% of SiO₂, 5 to 20% of La₂O₃, 5to 20% of Gd₂O₃, 2 to 35% of ZnO, 0.5 to 15% of Li₂O, 0.5 to 15% of ZrO₂and 0.2 to 10% of Ta₂O₅, containing 0 to 15% of WO₃, 0 to 8% of Y₂O₃, 0to 8% of Yb₂O₃ and 0 to 1% of Sb₂O₃ as optional components, and furthercontaining Nb₂O₅, BaO and GeO₂ as optional components, the total contentof the above components being at least 95%, the optical glasssubstantially containing none of PbO and Lu₂O₃, having a glasstransition temperature of 630° C. or lower, and having a refractiveindex nd and an Abbe's number νd which satisfy all of the followingrelational expressions, 1.80<nd≦1.90 35<νd≦50, and nd≧2.025−(0.005×νd).4. An optical glass as recited in claim 1, 2 or 3, which contains La₂O₃,Gd₂O₃, Yb₂O₃, Y₂O₃ and Sc₂O₃, the total content of La₂O₃, Gd₂O₃, Yb₂O₃,Y₂O₃ and Sc₂O₃ being 12 to 32 mol %, the molar ratio of the content ofLa₂O₃ to said total content being 0.35 to 0.66.
 5. A preform forprecision press molding which is made of the optical glass recited inany one of claims 1 to
 4. 6. An optical element which is made of theoptical glass recited in any one of claims 1 to
 4. 7. A process for theproduction of a preform for precision press molding, which comprisesflowing a molten glass made of the optical glass recited in any one ofclaims 1 to 4 from a flow pipe, isolating molten glass having apredetermined weight, and shaping the isolated molten glass having thepredetermined weight while the isolated molten glass is in a softenedstate.
 8. A process for the production of an optical element whichcomprises heating a preform made of an optical glass to soften thepreform and producing the optical element from the softened preform byprecision press molding, said preform being the preform recited in claim5 or the preform produced by the method recited in claim 7.